1. Field of the Invention
The present invention relates in general to a granular product particularly, but not exclusively, for use as an additive for powdered detergents.
More particularly, the invention relates specifically to granulated sodium silicate having a specific weight comparable with that of the usual components, more usually used in powdered detergent compositions.
2. Description of Related Art
It is well known that sodium silicates have been used intensively in the detergent industry as basic components of soaps since long before the advent of synthetic surfactants, particularly because of their well known sequestering properties towards magnesium and calcium.
Furthermore, they act as anti-corrosion agents and are effective in preventing the deterioration of washing machines, particularly the metal parts thereof.
Commercial alkaline silicates are generally characterised by their SiO.sub.2 :Na.sub.2 O ratio in the molecules. Silicates used in detergent compositions have a ratio greater than 1, generally about 2.4.
An alkaline silicate with a 1:1 ratio, that is to say sodium metasilicate, is considered too corrosive to be used safely and widely in detergent compositions for domestic use. Instead, for this purpose, it is usually felt necessary to replace the sodium metasilicate by an equivalent quantity of sodium silicate.
With regard to liquid compositions, generally used for washing-up liquids, the use of sodium silicate does not present any difficulty while a similar use in powdered detergents has recognized problems which until now have had to be accepted.
Dry sodium silicate, in fact, because of the way in which it has been manufactured until now, has a relatively low specific weight of the order of 0.2-0.4 and, in any case, considerably less than the specific weight of the dry components with which it is combined in the overall formulation of powdered detergents. For this reason, during storage or during any period in which the packaged detergent composition is at rest, it tends to segregate, substantially reducing the desired uniformity of behaviour of doses taken from different points in the package.
In order to overcome this problem it has been thought to manufacture the other components with equally low specific weights but this device has resulted in the sale and use of very light powdered detergents. The large volumes of these compositions has involved considerable transport, storage and packaging costs etc.
The methods of drying the sodium silicate used until now have been carried out with the use of spray towers supplied with solutions with low silicate concentrations (generally 20 to 30% dry content) or with rotary drum dryers which are supplied with solutions with higher concentrations.
In both cases, it has not until now been possible to obtain dry sodium silicate with a specific weight greater than the value of 0.4 mentioned above, this being particularly due to the fact that, when subjected to a drastic heat drying treatment, the silicate explodes in a phenomenon known as the "popcorn" effect and the flakes generated by such explosions incorporate air and thus have very low apparent densities and hence low specific weights.
Various attempts have been made to increase the "weight" of sodium silicate granules mainly with the use of rotary drum dryers. These attempts essentially provide for the periodic wetting, during the drying, of the silicate granules being formed obtaining what is termed in this branch of the art, recompaction of the silicate powder. The granules which are finally discharged from the dryer, along with a satisfactory grain size, have an appropriate specific weight, that is to say about 1, but are entirely insoluble. This negative aspect would seem to be due to the necessarily prolonged contact of the granules with the heating means with the consequent thorough dehydration of at least a surface layer of the granules with the elimination of the water of crystallization of the sodium silicate therefrom. The consequent vitrification of this surface layer renders the entire granule insoluble.
The problem underlying the present invention is that of providing a sodium silicate which can be obtained by a method which can be carried out on an industrial scale and which has physico-chemical characteristics such as to satisfy current requirements in the detergent field, as specified above, and which at the same time overcomes the problems mentioned with reference to the prior art.